Red close button not always working in browser

Whenever I open multiple browser windows (not tabs), lets say 5 or 6 different web sites for example, when I try to close all of them by clicking the red close buttons (X) in succession, several usually do not close unless I click inside the window and try to close again.

It seems that the Mac OS is losing the focus even though the X does appear when I point to each of the close buttons (before I click them).

I have reinstalled Mac OS X and applied all the updates but the problem still persists.

This behavior happens only with browsers. I have tried Safari, Chrome, and Firefox with the same results. It doesn't happen if I open several different programs like Picasa, iTunes, Finder, etc. It also doesn't happen if I open multiple MS Word or Excel windows, only browser windows.

I have repaired permissions but still no joy.

Does anyone have any advice as to what I might try next? I searched these forums but could find no one with this problem.

Thanks in advance for the help.